Conventionally, an occupant restraint device is known which restrains a vehicle occupant by an air bag or the like in the event of vehicle collision. For example, disclosed in Japanese Patent Unexamined Publication No. 2002-264747 is a structure in which a camera for photographing a vehicle occupant or the like is used as an occupant's state estimating device for estimating the state of an occupant and then an occupant restraint device such as an airbag is controlled based on the state of the occupant estimated by the occupant's state estimating device.
In an occupant restraint device of the aforementioned type for restraining an occupant in the event of a vehicle collision, a technology for obtaining precise information such as the posture and/or the body size of a vehicle occupant in a vehicle seat by using cameras is highly demanded. For example, with regard to the body size of vehicle occupant, there may be a difference between the fact and appearance due to the thickness of clothing and the like. Under this circumstance, it is required to know the actual body size of the vehicle occupant precisely. However, though the aforementioned patent document discloses the idea of estimating the state of the occupant by using a detection device, there is no description about concrete arrangement effective for grasping the posture, the body size, and the like of the vehicle occupant precisely.